The invention concerns an electrode assembly for glass tank furnaces with a rod-shaped electrode, inserted in a cylindrical electrode holder, through which it can be advanced, the electrode holder being provided with a device for forced cooling along at least part of its length, whereby the holder is installed in an electrode block with the face of the holder installed below a level determined by the design of the furnace and whereby a seal is provided between the electrode holder and the electrode outside the furnace.
It is known that rod-shaped electrodes, which are normally made of molybdenum, high alloy steels or inconel, can be inserted into the glass melt either through the furnace superstructure, crown, sidewalls or bottom to provide direct heating of the glass melt by means of the Joule effect as a result of the current passage through the glass melt. In the majority of cases the electrode material is molybdenum.
In the furnace atmosphere, which contains a substantial proportion of Oxygen, temperatures of up to approximately 1600.degree. C. occur, depending on the glass composition. At such temperatures most electrode metals tend to oxidize significantly which soon leads to their destruction. Molybdenum in particular oxidizes to an appreciable extent at temperatures above approximately 550.degree. C.
The part of the electrodes immersed in the glass melt does not suffer the same level of oxidization, as the glass melt itself exercises a certain degree of protection against oxidization. Therefore the oxidization problem is only present here to a minor degree.
Patents DE-B-24 25 025 and U.S. Pat. No. 5,125,002 teach that, in the case of electrodes which are inserted into the melt through the bottom of the furnace, the part of the electrode which is outside of the melt must be surrounded by a cooling jacket and that an annular gap must be left between the electrode and its surrounding electrode block, so that the glass can penetrate into this gap where it solidifies in order to produce a sealing effect. In order to advance the electrode the glass surrounding the electrode must be melted by reducing the cooling. However, this results in a temperature increase on the part of the electrode which does not come into contact with the glass which once again increases the risk of oxidization. In order to reduce this risk, it is suggested in the two above-mentioned patents that a seal is provided at the end of the cooling unit opposite the glass melt, and that the intermediate space between the cooling unit and the electrode is filled with an inert gas under pressure. However, the continuous supply of inert gas is a complicated process especially as glass tank furnaces normally have a number of such electrodes. Furthermore, the continual temperature changes which are necessary to solidify the glass on the one hand and to melt it on the other, result in significant thermal loading of the furnace bottom and the electrode holder. However, large temperature changes should always be avoided in glass tank furnaces.
Similar electrode assemblies have also been produced for the sidewalls of glass furnaces. Such assemblies are described in DE-AS 10 15 989 and in DE-OS0-29 18 643. In both installations glass is also used as a sealing material and must be melted before the electrodes can be advanced.
However, the assemblies described above can only be heated to temperatures of approximately 1000.degree. to 1050.degree. C. as above these temperatures the stability of the holder is reduced so much that the holder is destroyed when the electrode is advanced. However, there are glasses which have softening points above the temperatures mentioned, so that in such cases the cooling of the holder cannot be interrupted.
DE 26 21 380 A1 describes an electrode assembly with a system of rods arranged radially, whereby the rods are referred to as primary electrodes, in which the parts of the rods which penetrate the surface of the melt are provided with cooling jackets, which do not serve as holders, in order to prevent damage caused by oxygen from the atmosphere. However, no provision is made for adjusting the rods within the cooling jackets and the cooling jackets are not sealed against the rods. The actual electrode holders are situated above the surface of the melt.
DE 38 15 558 A1 describes an electrode holder of similar type, which is inserted horizontally in the furnace wall. As a result of the unavoidable gap between electrode and holder and with liquid glass it is inevitable that glass entering the gap will displace the air up to the non-adjustable seal and will solidify in the cooled area, whereby the cooling effect decreases from the inside to the outside. The advancing of the electrode is adversely affected. The point at which the solidification starts depends on the cooling intensity. In any event, glass under hydrostatic pressure serves as a sealing medium.